Steel materials such as slabs or billets are placed into a heating furnace prior to hot plastic working (for example, hot rolling or hot forging) and subjected to a specified heat treatment. Heating furnaces of the walking beam conveyor type have skid beams (fixed beams and movable beams) adapted to be internally cooled with water and arranged longitudinally of the furnace. The skid beams have attached thereto heat-resistant alloy blocks (skid buttons) arranged at a predetermined interval and serving as hearth metal members. The steel material placed into the furnace is transported within the furnace as supported by the skid buttons on the fixed beams and those on the movable beams alternately.
The hearth metal members must have oxidation resistance so as to be free of corrosion (oxidation wear) due to the high-temperature oxidizing internal atmosphere of the furnace, and such resistance to compressive deformation that the members will not readily deform even if repeatedly subjected to the compressive load of the heavy steel material to be heated. The materials conventionally used for hearth metal materials include high alloy steels such as high Ni-high Cr alloy steels (JIS G5122 SCH22, etc.) and Co-containing Ni-Cr alloy steels (e.g., 50Co--20Ni--30Cr--Fe). Also proposed as improved hearth alloy materials are 0.3-0.6% C--40-60% Ni--25-35% Cr--8-15% W--Fe alloys (Japanese post-examination publication SHO54-18650), 0.2-1.5% C+N--15-60% Ni--15-40% Cr--3-10% W--Fe alloys (Japanese post-examination publication SHO 63-44814), 1.0% .gtoreq.C--26-38% Cr--10-25% W--Ni alloys (U.S. Pat. No. 3,403,998), etc. Some of these alloys are already in actual use.
The operating temperature of steel material heating furnaces is elevated year after year for the treatment of a wide variety of steel materials, improvements in the quality of treated materials and savings in energy. It is common practice to operate the furnace at a high temperature of 1250.degree. C. or higher, and the internal furnace temperature is likely to exceed 1300.degree. C. Higher oxidation resistance and improved resistance to compressive deformation are required of the hearth metal members in order to carry out the high-temperature operation efficiently and safely.
However, the conventional heat-resistant alloys fail to fully withstand such high-temperature operations. Although it may be attempted to cool the hearth metal members more effectively by the internal water-cooling structure of the skid beams, the attempt leads to an increased heat loss due to the cooling water and uneven heating of the steel material to be treated as supported by the hearth metal members (occurrence of so-called "skid marks") and can not be a substantial countermeasure.
An object of the present invention is to provide a heat-resistant alloy steel having improved high-temperature characteristics in order to solve the above problem encountered with hearth metal members.